What will be the focal length of a convex lens, which produces a virtual image at a distance of $30\mathrm{cm}$ of an object placed $15\mathrm{cm}$ in front of it.

Why is a convex lens called a convergent lens?

The following diagram shows the use of an optical device to perform an experiment of light.

As per the arrangement shown, the optical device is likely to be a _____.

Given below are two statements : one is labeled as Assertion $A$ and the other is labeled as Reason $R$.

Assertion $A$: For a simple microscope, the angular size of the object equals the angular size of the image.

Reason $R$: Magnification is achieved as the small object can be kept much closer to the eye than $25 \mathrm{cm}$ and hence it subtends a large angle. In the light of the above statements, choose the most appropriate answer from the options given below:

What are the types of spherical lenses? Describe the principal focus, aperture and the optical centre of a spherical lens.

What is meant by the optical centre of a convex lens?